Seed: Forest

Seed Map

You are going to spawn in the ocean, perhaps on a survival island. You will need to teleport to the island; I personally jumped to the stronghold coordinates , and landed just off the coast. Note that there is a mainland/small continent not far to the east, as well.
The Seed is "Forest", with a capital F, no quotes. You can also type 2110048317

Skeleton Dungeon

Spider Dungeon

This spider dungeon is much deeper within the same cave. Beneath the mushroom island is a massive cavern system that I didn't even begin to explore, I'm sure. Note the highlighted chest was buried under gravel, for me.